The present invention relates to an image forming apparatus using electrophotography such as a facsimile, a printer, a copier and the like.
A general image forming apparatus using electrophotography includes a fixing unit that fixes a toner image to a sheet by application of heat and pressure. The fixing unit includes a fixing roller having an internal heat source and a pressure roller pressed against the fixing roller. The sheet to which a toner image is transferred is fed through a nip portion between the fixing roller and the pressure roller. When a print command is received, the image forming apparatus starts rotating the fixing roller at the same speed as a printing speed, controls a temperature of the fixing unit, and feeds the sheet through the fixing unit so as to fix the toner image to the sheet.
The fixing unit generally includes temperature sensors for detecting temperatures of the fixing roller and the pressure roller. When the sheet starts to be fed toward the fixing unit, the heat source starts heating the fixing roller. As the fixing roller is heated, a heat storage amount gradually increases. Generally, the heat storage amount reaches a sufficient amount for fixing the toner image when the sheet reaches the fixing roller.
In this regard, when the thickness of the sheet is thin, the temperature of the fixing roller overshoots and finally reaches the target temperature. Therefore, it is necessary to provide a waiting time before starting the feeding of the sheet. In this regard, Japanese Laid-Open Patent Publication No. H10-104990 discloses a configuration capable of reducing the waiting time.
However, in the general image forming apparatus, it is difficult to obtain excellent fixing property.